WARNING for references to violence and torture
Title from Heaven Knows by The Pretty Reckless
Archangels are not made for cages.
Sam knows this intimately, remembers the fury and the power of two of them locked away, how they had screamed and burned, how his eyes had burnt out and his ears dripped blood. Their rage had been enough to drown out the screams of damned souls, so that the only ones left were Sam's, alone in their own personal Hell.
Michael didn't last long after the fall. A century, perhaps, two at most. Sam can't know for sure - down there, time had no meaning. There was no morning, no night. No turn of the seasons, no clock ticking on the wall. Just... endlessness.
Sam remembers how, at first, Michael and Lucifer had fought between them, and that alone was torture enough. But it hadn't been long before they turned their ire on Sam, the one responsible for them being here. Sam has tried to forget that time, because an archangel has the power to level a city with the snap of their fingers, but now that power was confined. And the things they wrought together... There can be no words.
Eventually, Michael's attacks dropped off. Sam doesn't know why; Lucifer always sniggered that he'd gone mad, after being in Hell for so long, and Sam supposed it was plausible enough. He had, after all.
Lucifer, though, Lucifer was in his element. He had been caged before, but Sam had no illusions that the second time was like the last. The first time, the Cage had been just that; this time, it was his kingdom, and Sam was his prisoner to do with as he wished. He had made Sam scream and beg and bleed, but it was never enough for him. Lucifer always wanted more, kept coming up with bigger and more painful tortures, and it seemed there was no limit to his creativity or hatred. They could have been there forever, and still Lucifer would never stop.
And Sam knows that, should Dean lock himself with Michael, there would be no way of rescuing him. Sam had barely endured a thousand years or so with Lucifer; Dean would have to endure so many more. And this Michael was far stronger than the Michael in the Cage; he would not break under imprisonment. He would destroy Dean in every way possible, and then some. Just because he could. Just because there was no one around to stop him.
Archangels are not made to be imprisoned; Sam learned this lesson a long time ago. And he will do everything in his power to make sure Dean will never have to.
Gotta tell you, folks, this season is not proving inspiring to me! I hope you enjoyed this all the same.